Antacid preparations for oral administration and reduction of gastric acidity have long been known. Gastric antacids are generally classified as absorbable and nonabsorbable, depending on the amount of systemic absorption of the cation responsible for the neutralization of gastric hydrochloric acid. The preferred gastric antacids are compounds whose cationic portion is not absorbed from the intestine and that raises the pH of the gastric contents only to about 5. This class of compounds is referred to as nonabsorbable buffer antacids. Aluminum compounds are generally considered nonabsorbable. More recent studies, however, indicate at least some systemic aluminum absorption does occur. Aluminum compounds have many of the desirable properties of an ideal buffer antacid. These include a good buffering ability, the absence of acid rebound and the absence of gas producing components. Nonaluminum antacids do not possess all of these desirable properties of aluminum compounds and are therefore less desirable as antacids.
The effectiveness of aluminum containing antacids for the treatment of peptic ulcers has been clinically documented and is widely accepted. There are, however, certain side effects of using this type of antacid of which constipation is the most common.
Constipation results when aluminum inhibits contraction of the smooth musculature delaying stomach emptying thus resulting in slight blockage. In addition, aluminum interacts in the intestine with anions such as phosphate and carbonate and forms insoluble compounds. The biological consequences of the interaction of aluminum with phosphate following the intake of aluminum hydroxide are well known.
The primary event is precipitation of phosphate in the small intestine leading to elevated fecal phosphate. This reduces the availability of phosphorus for systemic absorption and may lead to a lowering of serum phosphate levels. Because of the importance of phosphate, the body has a series of homeostatic mechanisms to overcome this reduction in available phosphate. Under conditions of normal phosphate levels, the input into serum is balanced by its rate of removal. The sources of input are intestinal absorption, bone mineral resorption, renal phosphate reabsorption and transfer from soft tissue. Sources of removal include intestinal secretion, bone mineralization, glomerular (kidney) filtration and transfer to soft tissue. When any one of these factors is perturbed, the other processes are modified to restore balance. Thus, when available phosphate decreases, processes such as bone resorption and renal reabsorption increase considerably. The bone resorption process, in addition to supplying phosphate, also resorbs calcium leading to elevated urinary calcium excretion. Thus, indirectlY, aluminum interferes with calcium balance by causing calcium loss and it has been suggested that the demineralization process may be accelerated by aluminum.
The following clinical changes have been reported, in normal patients, in which the effects of aluminum on mineral metabolism have been studied.
1. Increase in fecal phosphorus and calcium.
2. Decrease in serum phosphorus.
3. Decrease in urinary phosphorus.
4. Increase in urinary calcium and magnesium.
5. Increase in renal reabsorption of phosphorus.
The combination of these biochemical perturbations is considered to be a contributing factor in bone loss.
A large number of commercial antacids are available which contain aluminum compounds. Most of these products contain aluminum hydroxide alone or in combination with other basic compounds such as magnesium hydroxide, calcium carbonate, sodium carbonate and the like. Other aluminum containing compounds include aluminum phosphate, dihydroxy aluminum aminoacetate, aluminum carbonate, and magaldrate, a chemical combination of aluminum hydroxide and magnesium hydroxide. None of these products overcome the undesirable mineral metabolic side effects of aluminum containing antacids.
It would, therefore, be desirable to develop an antacid composition having the advantageous properties of aluminum antacids while minimizing the undesirable side effects.